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MMAB Frequently Asked Questions

Q: How can I find out about system changes, product changes, and experimental products?
A: We have a low-volume mailing list for such announcements. Please visit http://polar.ncep.noaa.gov/mmab/mailing.shtml to sign up.
Q: When are the products made available?
A: In general, as soon as possible after construction.
Q: Are there any alternate locations I can get the files from?
A: Absolutely. Polar is not an operational system itself:
Q: Polar is not letting me in to ftp files, do I need a special account or password? Have you discontinued public service?
A: There are no special accounts or passwords required to access our products. If public access were ever discontinued, the fact would be mentioned prominently on the web site. What probably happened to you is that you have a firewall in place now that is not identifying itself correctly. If your apparent address does not respond to an identity query, ftp connection will be refused. Please talk to your system administrator about how to fix this problem on your side.
Q: I'm trying to retrieve files, have a correctly established firewall, and polar is still refusing service after the initial login.
A: For some connections, the 'passive' setting is important and may need to be toggled.
  • ncftp (ncftpget) may need the -E option.
  • ftp may need you to issue the "passive" command (this will flip your current setting, whatever that is)
  • Some ftp software, command line ftp from Windows XP is known to have this problem, produces failures if you attempt to open multiple connections.
Q: Can I have the products emailed to me?
A: Yes. Please see http://weather.noaa.gov/pub/fax/ftpmail.txt for the instructions.
Q: I would like to link to your site, or to images, is that ok? Is there anything special I need to do, permissions to be obtained?
A: It is not only permitted but welcome for people to link to our products. It is generally better to link to the html pages than to the graphics themselves, as we change graphic file names more often and graphics don't lend themselves to messages of impending changes as html pages do. Please see http://www.weather.gov/disclaimer.php for the NWS disclaimer on product distribution, which includes the details. You may find it better for your site's appearance, when it comes to the graphics, to construct your own, as many sites do. The model and analysis output are available for ftp for all our operational products. See the web pages for those products for the details of where to ftp them from.
Q: I hear that polar won't be around much longer.  What is its future and its replacement?
A: A machine named polar is expected to be around for some time to come. What hardware stands behind that name can be expected to change, as has already happened 4 times in the history of the name. See also the question about where else MMAB products may be found.

Acronym Questions

Q: What is 'Z' (as in 00Z)?
A: Commonly referred to as Zulu Time, it is a legacy reference in the National Weather Service, carried over from military usage. For most practical purposes it is the same as Coordinated Universal Time (UTC)
Q: What is UTC?
A: Coordinated Universal Time, the worldwide time standard. For precision use, there is actually more than one UTC.
Q: What is GRIB?
A: GRIdded Binary. Standard format for meteorological information specified by the World Meteorological Organization
Q: How do I read GRIB?
A: GRIB comes in two formats, GRIB1, for older files, and GRIB2, for newer files. GRIB2:
http://www.cpc.ncep.noaa.gov/products/wesley/wgrib2/
ftp://ftp.cpc.ncep.noaa.gov/wd51we/wgrib2/
GRIB1:
http://www.cpc.ncep.noaa.gov/products/wesley/wgrib.html
ftp://ftp.cpc.ncep.noaa.gov/wd51we/wgrib/
Q: What is BUFR?
A: Binary Universal Form for the Representation of meterological data. A WMO standard binary format.

Science Fairs

Q: We don't generally provide ideas for science fair projects, but one question did arrive and was suitable for our branch to comment on. It pertained to constructing a science fair project to examine how waves are formed.
A:

If you have a small lake nearby (a few hundred yards to a couple of miles across), this would be easiest and give the best data. The thing to do is measure the wind speed (science fair books at the library often have directions for build-your-own anemometers, or you can use your local weather reports) and then measure the wave heights as you move away from the shore in the direction the wind is blowing. Very near shore, the waves will be small. As you go off shore (but in the wind's direction) the waves will build up, even though the wind is no faster than before. On a lake I regularly visit, it is easy to see this effect across the half mile of the lake's length. There may even be no apparent waves for the first 100 yards, if the wind speed is low enough. The distance from shore is called the 'fetch'. If your lake is small, then you may find that you're 'fetch-limited' -- that is, the waves are continuing to get taller even at the far shore.

The harder method is to build a combination wave tank/wind tunnel. Since you probably can't build a wave tank a half mile long, you won't be able to explore the fetch effects. But what you should be able to examine is the wind speeds for the start of wave growth and what effect the initial state of the water has. Wave tanks can be interesting to study in their own rights (wave shadows behind pillars, waves meeting other waves and interfering or reinforcing each other).

The best wave tank is one made with see-through sides. Plexiglass is good for this. You'll want to be able to change the depth of water in the tank (this may have an effect on your results, I don't know so it is good to test) so ideally the tank should be a foot or two tall. For wave generation by wind, you don't need a very wide tank, a foot should be enough (to do other wave experiments, you would make a tank that was only a few inches deep, but was easily two feet wide, and preferably three). For wave generation, you'll need some length to the tank, at least 3-4 feet. This gets to be a lot of water, which will weigh a lot and take a fair amount of time to fill. The ends of the tank, you'll need to make a fair amount of time to fill. The ends of the tank, you'll need to make in such a way (hinges? I'm not the best hands-on construction person so find someone who is good to figure out the best way) that the tank ends don't block the entry of air to the tank. You want the end the air starts from to be barely above the water level of the tank. (The far end doesn't have to match so well.)

The wind speed is a bit of a problem. A home built wind tunnel may not be able to reach a high enough speed to build good waves. What you may find easier, if you have a fairly unobstructed place to set up the wave tank, is to build the wind tunnel-like cover and then let the natural wind funnel in through that. What you'll want is a cover that does funnel, the wider the open end is, the better, the wind in to your wave tank. Then keep a constant size to the chamber over the tank. The funnel will speed up the wind (by making the same amount of air pass through a smaller channel) in proportion to the area ratio. That is, if the big end facing in to the wind is 10 times the area of the channel over the water, then the wind in your tunnel should be about 10 times faster than the outside world.

To work with a wind tunnel that you control the whole way, what you'll need is a strong fan to _pull_ the air in to the tunnel. Ideally, the fan will be about the size of the tunnel where the tunnel is over the wave tank. If you have to widen out to reach the fan, then you'll lose some of the area effect for making the winds faster. But you do have to make the covering extend out to the fan, or you won't pull air through the tunnel. You'll also need an anemometer to tell how fast the wind is moving.

There is room for a lot of experimentation on how to construct this, and on what to measure. The above are just some starting points.


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Page last modified: Monday, 17-Dec-2012 19:48:48 UTC